How hot should an SSR run, (and is this a sign of a DOA one?)

[shortyjacobs]

I suspect that most of you complaining about cheap Chinese SSRs failing don't have a large enough heatsink or are not using thermal compound between the SSR and heatsink.

A SSR will dissipate about 1 watt for every amp its controlling. So, 23 amps for a 5500 watt element = about 23 watts of power and every bit of it is heat. The heat is generated because the heart of a SSR is a semiconductor and semiconductors never turn all the way on.

And as far as heat sinking goes, if you use a good heat sink with 1" fins you will need a heat sink that's about 2" X 3" for 10 amps. This is assuming the heatsink is mounted on the outside of your box with the fins oriented so that air moves up between them. Mont the SSR inside a box and there is no chance for airflow without a strong fan. Pull 23 amps and your heatsink needs to be at least 2.5X that size or 5" X 3" to cool your SSR.

When the commercial guys push power through a SSR they usually go with one of these heatsinks. It will be mounted so that air drafts through like a chimney or they will blow ait through the heatsink.
Heat Sink, Aluminum - Relay Accessories - Relays - 6CXA8 : Grainger Industrial Supply

The thermal compound is the same type of thermal "grease" used to prevent PC CPUs from overheating. It fills in the microscopic spaces between the SSR & heat sink and without it your SSR will run hot, even on a proper heatsink. Here is some thermal compound at a good price.
BestByte.net - Discount Computer Hardware & Reliable Service: Thermal Compounds & Adhesives

I don't know if mine failed BECAUSE it got hot, or it got hot BECAUSE it failed...

I know for sure that it had semi-failed, (worked fine when cool, but it got REALLY hot, and then latched open). I know I was getting good conductivity to my heatsink, because the heatsink was VERY hot.

In any case, I have the new SSR, and I mounted a fan to blow across my (much smaller than you show), heatsink. The heatsink barely gets warmer than room temp now, and the SSR functions as expected.

I think it's a definite possibility that I fried the SSR by not using a fan on the wimpy heatsink, and it's also a definite possibility that the SSR was DOA, and it's failure mode was to heat up a helluva lot.

Either way, I'm happy now!

 

[passedpawn]

I suspect that most of you complaining about cheap Chinese SSRs failing don't have a large enough heatsink or are not using thermal compound between the SSR and heatsink.

A SSR will dissipate about 1 watt for every amp its controlling. So, 23 amps for a 5500 watt element = about 23 watts of power and every bit of it is heat. The heat is generated because the heart of a SSR is a semiconductor and semiconductors never turn all the way on.

And as far as heat sinking goes, if you use a good heat sink with 1" fins you will need a heat sink that's about 2" X 3" for 10 amps. This is assuming the heatsink is mounted on the outside of your box with the fins oriented so that air moves up between them. Mont the SSR inside a box and there is no chance for airflow without a strong fan. Pull 23 amps and your heatsink needs to be at least 2.5X that size or 5" X 3" to cool your SSR.

When the commercial guys push power through a SSR they usually go with one of these heatsinks. It will be mounted so that air drafts through like a chimney or they will blow ait through the heatsink.
Heat Sink, Aluminum - Relay Accessories - Relays - 6CXA8 : Grainger Industrial Supply

The thermal compound is the same type of thermal "grease" used to prevent PC CPUs from overheating. It fills in the microscopic spaces between the SSR & heat sink and without it your SSR will run hot, even on a proper heatsink. Here is some thermal compound at a good price.
BestByte.net - Discount Computer Hardware & Reliable Service: Thermal Compounds & Adhesives

Good advice all around. I used Arctic Silver 5, which is a premium (IOW costs more) paste for mating processors and heat sinks. The overclockers know what I'm talking about But you could get comparable (probably for free) from any computer store - every processor ships with a bit of it I think.

 

[thargrav]

I don't know if mine failed BECAUSE it got hot, or it got hot BECAUSE it failed...

I know for sure that it had semi-failed, (worked fine when cool, but it got REALLY hot, and then latched open). I know I was getting good conductivity to my heatsink, because the heatsink was VERY hot.

In any case, I have the new SSR, and I mounted a fan to blow across my (much smaller than you show), heatsink. The heatsink barely gets warmer than room temp now, and the SSR functions as expected.

I think it's a definite possibility that I fried the SSR by not using a fan on the wimpy heatsink, and it's also a definite possibility that the SSR was DOA, and it's failure mode was to heat up a helluva lot.

Either way, I'm happy now!

Most likely you fried the SSR. And it would run hot because the part was damaged and dissipating more wattage.

And BTW, the heat sink specs I was quoting is for free air & no fan. As you already know you can go smaller when using a fan to blow air across the heat sink. How small, I don't know. CPU coolers do this all the time.

 

[kazikame]

OK, a few things.... likely, your PID is not providing a sinking current to shut off your ssr. You need to jump a 10k resistor across the positive and negative output from your pid. This will pull your input to the ssr LOW when it is off. That being said, the ssrs still can stick when they get too hot, so you need to up your cooling capacity. put a little computer fan on the bottom of your box

 

[shortyjacobs]

Lol, thanks champ. This post was 9 years ago, it was just a failed SSR. Put in a new one, put some cooling on it to stop it from cooking itself, and it’s been fine for 9 years now.

 

[doug293cz]

OK, a few things.... likely, your PID is not providing a sinking current to shut off your ssr. You need to jump a 10k resistor across the positive and negative output from your pid. This will pull your input to the ssr LOW when it is off. That being said, the ssrs still can stick when they get too hot, so you need to up your cooling capacity. put a little computer fan on the bottom of your box

PID's designed to drive SSR's do NOT require any additional components (like resisters) between the PID and SSR (assuming you have the correct SSR type.)

Brew on